Underwater transient sound generators have been known and are used extensively to determine the profile of the sub-bottom terrain in underwater surveying. The present invention is concerned with an improved underwater transient sound generator of the kind which employs a piston plate which is repelled outwardly into the water by means of eddy currents induced in the plate by an energized coil. One example of this type of device is disclosed in U.S. Pat. No. 3,993,973 entitled "Underwater Transient Sound Generator Having Pressure Compensating Fillet", which issued on Nov. 23, 1976, of which I was a co-inventor.
Essentially, the invention disclosed in this prior U.S. patent consists of a structure for the improved tracking of the pressure at the rear face of the piston with the pressure at the front face of the piston. In this manner, as the device was raised or lowered in depth in the water, the ambient pressures on the front and rear faces of the piston would vary identically, resulting in the pressure pulse produced by the piston remaining constant over varying depths of submergence of the device.
However, there are a number of undesirable limitations of the invention disclosed in this prior patent. The prior invention has a maximum operating depth, which occurs when the rear diaphram is pressed against the fillet, thereby transferring all of the compressed gas to the non-compressible gas space at the rear face of the piston. Any further increase in depth will result in a change in the ambient pressure between the front and rear faces of the piston, and undesirable changes to the acoustic pulse signature.
The maximum depth of operation of the prior device can be extended by either of two methods: by increasing the initial volume of the compressible gas space, or by decreasing the initial volume of the non-compressible gas space. With respect to increasing the volume of the compressible gas space, because an additional volume in the compressible gas space equal to the volume of the non-compressible gas space is required for each additional atmosphere of ambient pressure, increasing the compressible gas space is not practical for deep water sound generation, as the volume required would result in a large and awkward device. A further disadvantage of this solution is that as the volume changes, the bouyant forces on the submerged device change. This change can result in the device being unstable, with the acoustical pulse being poorly and erratically aimed as it is towed above the seabed.
Making the non-compressible gas space smaller, seems to present two advantages. Firstly, for the same overall size of the sound generator, increased depth of operation can be achieved, without any greater instability at depth. Secondly, and most importantly, the closer the piston is to the coil, the tighter the electromagnetic coupling between the driver and the coil, and thus the lower the initial effective inductance of the coil curcuit. Because the initial inductance is lower, a greater efficiency of conversion of electrical energy into mechanical (acoustical) energy is possible.
Unfortunately, other phenomena affect the piston dynamics, limiting the effectiveness of reducing the volume of the non-compressible gas space. The spring constant of the gas contained in this prior invention is proportional to the ambient pressure. The greater the depth, and thus ambient pressure, the stiffer the gas spring. It should be noted that the gas spring acts both to resist the movement of the piston outwards, and to return the piston to its normal position closely adjacent the coil. Therefore, at greater depth as the gas spring gets stiffer, the outward stroke of the piston becomes shorter, and the return velocity becomes greater for the piston. This change in stroke dynamics changes the nature of the acoustic pulse produced. If the electrical input energy is increased the piston stroke can be made longer but again the maximum velocity of the piston increases. As the higher velocity piston has a greater momentum, on the return stroke it will tend to overshoot its normal rest position and strike the coil. Striking the coil causes the piston to stop suddenly, creating a secondary and unwanted acoustical pulse. The secondary pulse creates unacceptable noise, altering the acoustical signature and rendering the underwater survey results difficult or impossible to interpret.
Another limitation on reducing the non-compressible gas space becomes evident when the input energy is considered. There are good reasons to operate the device at relatively high energy levels, where increasing the input energy is considered as increasing the voltage for the same capacitance. Firstly, at a higher input energy, a stronger acoustical pulse is produced, which increases the depth of the subsurface penetration of the seabed by the acoustical pulse. Secondly, the higher the input energy, the greater the efficiency of the electrical to mechanical energy conversion.
Unfortunately, as the input energy is increased, so to is the velocity of the piston on the outward and inward strokes. In the prior device, a gap exists between the rear face of the piston and the coil. The piston tends to oscillate about the rest position, after the stroke cycle. The greater the piston velocity, the greater the amplitude of the oscillation and consequently for a given gap, at a certain input energy the oscillation is sufficiently large to cause the piston to strike the coil, creating the unwanted secondary pulse.
As a result, the minimum gap between the piston and the coil is a function of the maximum input energy required to achieve the desired subsurface penetration. It has been found that this design limitation results in a much lower input energy than would otherwise be possible, if, for example, the thermal conductivity of the support body was the limiting design factor.
In summary, the prior art device cannot usefully be adapted to deep water soundings because to extend the depth of operation of the device results in a bulky and awkward device, or in the unwanted and destructive production of a secondary sound pulse. Further, the device is limited to a maximum input energy which limits the efficiency of the device and reduces its depth of subsurface penetration.
A further problem with the prior invention is that as a result of the energy released by the coil, the body of the sound generator has a tendency to heat up, causing thermal stresses which can reduce the life expectancy of the device.